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Sunday, 26 December 2010

Water Walkers

There goes Christmas. Now it's time to look to the future, maybe even a little further than New Year's Eve. Or not. In any case, whatever your hopes and dreams, your trials and burdens, your strengths and weaknesses, walking on water would be really cool. More or less half of these miraculous creatures could have 'Jesus Christ' thrown onto their name, but for them walking on water is not a miracle, it's simply life or escape from death. They just don't know how lucky they are! But then again, who does?

Grebes are water birds famed for their extravagant courtship dance, which culminates in the pair running gracefully across the surface of the lake in perfect coordination. It's rather wonderful. Actually, they are so well adapted to life on water, it's probably easier for them to walk there than on land! Here's a lovely video.

Dolphins can be trained to do what is know as a tailwalk, where they rear up on their tail and propel themselves forward. More fascinating is the fact that the Whale and Dolphin Conservation Society in Australia have seen some trying it out in the wild. It's believed that they learnt the skill from a female dolphin named Billie, who spent some time in a visitor attraction. Now the others are practising and practising to get it right. It seems to be a completely useless activity done purely for the fun of it, like dancing. People are interested because this is an example of culture amongst non-human animals. Imagine if female dolphins started choosing their fellas based on their ability to tailwalk! Would their tails become all chunky and muscular? Footage from the WDCSA can be found here.

One of the most famous water walkers are the Basilisk Lizards. They have flaps between their toes which are used to increase the surface area of their feet. They slap one foot straight down and out, creating a pocket of air around their foot which allows them to push off against the water. Then they sweep this back and toward their body to move forward. Then the foot is moved out of the water to start again. Meanwhile, the other foot is doing the same thing but at the opposite time, providing forward movement and an equal and opposite force to the left and right so they don't fall over. They can run like this at the impressive pace of 1.5 metres (4.9 feet) per second for about 4.5 metres (14.8 feet). After that they both sink AND swim. Spectacular footage here.

Also known as raft spider, dock spider and wharf spider. Clearly, this spider has something to do with water. Indeed, they spend their time at the edge of ponds and streams with a few of their legs stretched on the surface waiting for vibrations. They mostly eat insects but since they can also dive underwater, large ones also eat small fish and tadpoles.

Their legs are covered in water repellent (hydrophobic) hairs which push water aside and create little dimples on the surface. This is a use of surface tension, where the molecules of water are attracted to other molecules of water. Since molecules at the surface don't have any water molecules above them, they are attracted all the more to molecules beside and below them. With their light weight and long, hydrophobic legs, fishing spiders can use the dimples they create as oars to row across the water. When they want to go really fast they can smash a couple legs straight down, akin to the Basilisk Lizard. When they're particularly relaxed, they can lift a few legs into a breeze and sail along pleasantly. Sounds lovely.

It looks like some frogs can also jump across the water's surface for a short distance to escape predators. I didn't know that! And now I can't find any information about it. For all I know almost everyone in the world knew and no-one ever thought to tell me. Perhaps it's such common knowledge that no-one bothered to mention it anywhere. Maybe that's what I get for being a city boy. In any case, here's a video.

Water skaters are bugs that walk on water in a manner similar to the aforementioned fishing spider. Their legs have tiny hairs with even tinier grooves, as well as a waxy covering to ensure high hydrophobia. They skate and stride with their middle legs and are capable of moving at speeds of 1.5 metres per second after sensing the vibrations of some struggling insect. They have to have really long legs to get enough out of the surface tension and larger species have to have legs comparatively even longer.

Small snails are buoyant enough to float, they can walk not so much ON water, as under it. But how can they stick to the under-surface of water to actually walk in any particular direction? It's another use of surface tension. The snail's foot ripples, which, if you remember, is how snails walk in any case. This creates similar ripples on the water's surface and a downward force as it tries to become flat again. These ripples and forces are just enough for the snail to use as traction to slide without slipping.

The Brazilian Pygmy Gecko is tiiiiiny! Barely an inch or two long. One thing you might know about Brazil is that it has lots of rainforest and rain, so this gecko has to adapt lest it drown at almost every opportunity. It has hydrophobic skin, not just feet, and is light enough to just float on any puddle. No effort, no thought, just sitting back and being absolutely incredible. Now that's my kind of miracle! Check out the video.

In regards to the Basilisk lizard (I wonder how they got that name, since I'm pretty sure that they weren't a snake egg hatched by a rooster (or was that for the cockatrice and the basilisk was a rooster egg hatched by a snake?)), If you fill a sink or bathtub with water and quickly slap down a washrag (either wet or dry, both work, though wet gives it some weight) you can catch a pocket of air.